I solved the puzzle using the usual ways (innies and outies, etc., if you need the solution pse tell me), it's difficult but doable; i5 = 3, a4-b4 = [5,9], a6-b6 = [7,8] are inmediate; the first "unknown" number to place is e6 = 9: two possibilities, 6 or 9, considering that cage 30 must have 6,7,8,9 and that [7,8] are already in a6-b6; since e6 + h6 + i6 = 18 (outies), e6 = 6 >>> h6 + i6 = 12 with h6 = 3, i6 = 9 would be impossible due to a 9 in cage 29. Etc., it requires some time, small equations and analysis as usual but I think the level of difficulty is about the same as some of your book's puzzles. I can confirm the solution is unique.

I managed the "Daily No. 2456" at killersudokuonline.com with some difficulty. Even if the indicated difficulty level is only "hard" (not "mind-bending" or "outrageous") I found it clearly harder than the difficult killer sudokus here at calcudoku.org, which I am able to solve on-screen, memorizing candidate numbers when necessary and doing the calculations in my head. The "Daily No. 2456" I wouldn't have managed without using the option for entering candidate numbers on-screen because I found it too hard to keep all those intricate dependencies in my head.

It makes sense that the difficult puzzles in the last part of your new killer sudoku book would be at a "harder-than-difficult" level similar to that of the "Daily No. 2456" (as noted in clm's comment) because they are meant for being solved using a print (or the paperback) so puzzlers can enter candidate numbers in cells and make notes and calculations in the margins. The charming thing about book puzzles is the "armchair" feeling of solving them away from the computer and immersing yourself in pen-and-paper analysis.

I managed the "Daily No. 2456" at killersudokuonline.com with some difficulty. Even if the indicated difficulty level is only "hard" (not "mind-bending" or "outrageous") I found it clearly harder than the difficult killer sudokus here at calcudoku.org

Yes, so far the difficult killer sudokus on the site are clearly easier than the difficult ones of the book.

With a few more solver improvements I managed to get the solving time down by a factor of 20 Two more obvious improvements to go, after which I should be able to rate this puzzle better.

So in the end solving time went down from ~ 10 minutes to less than 1 second (!)

Congratulations! Well done!

pnm wrote:

Now I need quite a fundamental update to the way I determine a puzzle's difficulty as well...

Yes... I just went through the calendar, charting number of times solved against difficulty level for the Difficult Killer Sudoku. The scatter plot shows no correlation between the two. I realize other factors (such as day of week) may be in play, but with a more accurate difficulty rating, I would expect to see more times solved for easier puzzles and fewer times solved for harder puzzles.